A helicopter prepares to land on the China Maritime Safety Administration (MSA) ship Haixun-31 on Sunday during a brief stop in Sanya, in southern China's Hainan province. The ship is expected to join an ongoing search for the missing Malaysian Airlines' passenger plane that vanished on Saturday. (AP)

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The Malaysian Airlines flight that disappeared with 239 people aboard may have tried to return to Kuala Lumpur, a Malaysian Air Force official said Sunday.

Air force chief Rodzali Daud said military radar indicated that Malaysian Air Flight MH370 “may have made a turn back,” but did not say how far it got. “We are trying to make sense of this,” Daud said. The flight vanished early Saturday, two hours into a scheduled six-hour flight from Kuala Lumpur to Beijing.

The Wall Street Journal reported that a Vietnamese search aircraft located debris that could be from the missing plane floating in waters off southern Vietnam.

Military ships and aircraft from a half dozen nations continued searching for the Boeing 777 Sunday. The U.S. Navy has provided the USS Pinckney, a guided missile destroyer that carries two MH-60R helicopters, and a P-3C Orion with long-range search, radar and communications capabilities.

“Our thoughts and prayers go out to all of those affected by this tragic event,” the Navy said in a statement.

Two large oil slicks were found off the southern tip of Vietnam Saturday, but no signs of wreckage had been found Sunday.

Ignatius Ong, a representative of Malaysia Airlines, said at a press conference in Beijing that search and rescue efforts are “still unable to detect the whereabouts of the missing flight.”

The airline has been telling relatives “to expect the worst,” Ong said.

Authorities were investigating potential terrorism after discovering that two passengers apparently had been flying with stolen passports. Interpol, the France-based international policing agency, confirmed Sunday that the Italian and Austrian passports had been entered into its database after they were reported stolen in 2012 and 2013.

No country had checked the passports with Interpol since the thefts, both of which took place in Thailand, he agency said, adding that it was reviewing the passports of everyone listed on the flight manifest.

“Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in INTERPOL’s databases,” said INTERPOL Secretary General Ronald K. Noble.

The Italian Foreign Ministry said Luigi Maraldi, an Italian national, reported his stolen passport last August. Austrian officials said Christian Kozel’s passport stolen in 2012. Both names were ticketed to continue from Beijing to destinations in Europe and thus did not need visas for China.

When the plane is located, the airline will set up a command center either in Malaysia’s Kota Bharu, or in Vietnam, depending on its location. A response control center will be activated as close as possible to the incident area, he said.

The airline plans to send two family members for each missing passenger to Kuala Lumpur, or another destination if closer to the plane’s location. The airline is working with Chinese authorities to get passports for relatives who lack them, and with the Malaysian Embassy in Beijing to get entry visas for Malaysia.

The first flight will be Monday for relatives who want to travel to Kuala Lumpur, and arrangements will continue for those who decide to wait in Beijing, Ong said.

Speculation on the cause of the crash ranged from catastrophic mechanical failure to terrorism, fanned by a flight manifest which showed two passengers had flown with stolen passports.

Malaysian Transport Minister Hishammuddin Hussein said authorities are looking at four possible cases of suspect identities and had contacted the FBI and other intelligence agencies. “We do not want to target only the four; we are investigating the whole passenger manifest. We are looking at all possibilities.”

The 11-year-old jet was last inspected 10 days ago and found in “proper condition,” airline officials said. The lack of a distress signal from the pilots “suggests something very sudden and very violent happened,” said William Waldock, who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.

Weather was not believed to be a factor. Light rain and snow was falling over South and Central China, but it was well below the aircraft’s last known, 35,000-foot altitude.

However the flight disappeared, the mother of Philip Wood — one of three Americans on the flight — was resigned that he was gone. “You want to know how it feels to lose a son at the age of 50? It’s devastating,” Sandra Wood said. She saw her son, an IBM executive who worked in Malaysia, a week ago.

Freescale Semiconductor, an Austin-based tech company, said 20 employees from China and Malaysia were aboard. “Our thoughts and prayers are with those affected by this tragic event,” said CEO Gregg Lowe.

Search efforts were concentrated where two oil slicks, each 6 to 9 miles long, were found by about 80 miles south of the island of Tho Chu in the Gulf of Thailand by Vietnamese military aircraft Saturday.

Malaysia sent nine planes and 15 ships to search waters between Malaysia and Vietnam. The Philippines dispatched three Navy ships and a surveillance plane. China sent two ships. A U.S. Navy destroyer equipped with two helicopers is also assisting the operation.

The plane was two hours into its flight when Subang Air Traffic Control lost contact at 2:40 a.m. local time (1:40 p.m. Friday ET). It was scheduled to land in Beijing at 6:30 a.m. local time. The last radar signal was received as the aircraft approached Vietnam airspace near the Ca Mau province.

The twin-engine jet was carrying 227 passengers and 12 crew.

They’re from 14 countries, including 153 from China, 38 from Malaysia, seven from Indonesia, six from Australia, five from India, three from France, two each from New Zealand, Ukraine and Canada and sole travelers from Russia, Italy, Taiwan, Austria and the Netherlands. Besides Woods, the other Americans on the manifest are young children — Nicole Meng, 4, and Yan Zhang, 2.

Malaysia Airlines CEO Ahmad Juahari Yahya said that the company is working with emergency responders. “Our thoughts and prayers are with all affected passengers and crew and their family members,” Yahya said.

At Beijing’s airport, authorities posted a notice asking relatives and friends of passengers to gather at a hotel to wait for further information.

The flight had seasoned pilots, according to the airline. Capt. Zaharie Ahman Shah, 53, of Malaysia has 18,365 flight hours and has been with the airline since 1981. First Officer Fariq Ab.Hamid, 27, also of Malaysia, has 2,763 flight hours .

Boeing 777s have a strong safety record. Since their 1995 debut, they’ve been in only two major accidents.

The worst was last July, when an Asiana Airlines Boeing 777-200 with 291 passengers and 16 crew crashed on landing at San Francisco International Airport. Three passengers were killed — one by a fire rescue truck. There were serious injuries to 48 passengers. Pilot error is being investigated.

The search for the Malaysia Air flight comes amid one of the safer stretches of commercial aviation. In the U.S., 2012 was the industry’s safest since the dawn of the jet age. The last major airline disaster was in 2009, when an Air France Airbus 330 crashed during a flight from Rio de Janeiro to Paris, killing all 216 passengers and 12 crew.

Malaysia Air’s last air fatalities were in 1995, when a flight crashed near Tawau, Malaysia, killing 34.